SOLUTION: I'm getting stuck on reciprocal. I know that the opposite of -x is x but I can't remember or find the WHY we have to do that, Here is the simple problem I can't seem to figure ou

Question 552550: I'm getting stuck on reciprocal. I know that the opposite of -x is x but I can't remember or find the WHY we have to do that, Here is the simple problem I can't seem to figure out because I want to learn why I have to do the opposite.
21 - x = 28
Here's what I did:
21 + (-x) = 28
21-21+(-x) = 28-21
-x = 7
I think I'm supposed to read this as "the opposite of" -x is X but how do I get the correct answer of -7?
Thanks for your help,
Ramar

Answer by Alan3354(69443) (Show Source): You can put this solution on YOUR website!
21 - x = 28
----------
Add x
21 = x + 28
Subtract 28
-7 = x
x = -7

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